Always use the effective/horizontal length if the top tube is not parallel to the ground (i.e., sloping)--it is still measured cnter of head tube to center of seat tube. If the top tube is parallel to the ground, then the center-to-center top tube length and horizontal length are the same.

The new frame is 4mm longer, so all things being equal you'd want a 4mm shorter stem (effective length of the stem, parallel to the ground). Things that do not make this "all things being equal" include but is not limited to: head and seat tube angles; saddle to handlebar drop; reach, drop and width of handlebars...you get the point. 4mm is not that big of a difference.

Originally Posted by slopvehicle

Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.